Fingerprint recognition technology can be used for personal identification or verification. Specifically, in a fingerprint recognition system, fingerprint images are captured by a fingerprint sensor. Thereafter, a matching process between the fingerprint images captured and templates stored in the system is performed, so as to determine if they are matching or not. Since the fingerprint recognition technology is kindly to user and fingerprint is distinctive, the fingerprint recognition technology is widely used in various applications, such as personal identification in policeman stations, physical access control in buildings, device access control in personal computers or mobile phones, and so on.
The first step of fingerprint recognition is fingerprint image capturing, which can be achieved by many type of fingerprint imaging systems such as optical fingerprint imaging system, capacitive fingerprint imaging system, ultrasonic fingerprint imaging system and the like. Among the above, the optical fingerprint imaging system has better performance and lower cost.
An optical fingerprint imaging system can implement the fingerprint image capturing based on light transmission and reflection at an interface between optical mediums. Referring to FIG. 1, an existing optical fingerprint imaging system is illustrated, which basically includes a backlight board 101, an optical fingerprint sensor 102, and a piece of protective glass 103. During a fingerprint image capturing process, a human finger 104 touches the protective glass 103 for a while. Light 1011 (indicated by long arrows pointing up in FIG. 1) emitted from the backlight board 101 penetrates through the optical fingerprint sensor 102 and the protective glass 103, and then reaches the interface between the finger 104 and the protective glass 103. At the interface, part of the light 1011 is reflected while the other part passes through. Thereafter, reflected light 1012 (indicated by short arrows pointing down in FIG. 1) penetrates through the protective glass 103 again and reaches the optical fingerprint sensor 102. In the optical fingerprint sensor 102, the reflected light 1012 is converted into electrical signals, such that a fingerprint image can be established. The image is obtained by processing the electronic signals which is proportional to strength of the reflected light 1012. Since the light 1012 is reflected by the interface between the finger 104 and the protective glass 103, it can be conceived that the strength of the reflected light 1012 can represent features of the interface. Further, the features of the interface depend on the features of the fingerprint. In light of the above, the image can directly present the features of the fingerprint.
However, existing fingerprint imaging systems are only used for capturing fingerprint images, which means they have limited usages.